A circuit element such as, for example, a coil element (an inductor) has a frequency characteristic attributable to its material and structure, which deviates from an ideal inductor characteristic. Because of this, in order to accurately calculate a characteristic of a coil element by using a simulator or the like, an equivalent circuit model designed to exhibit an actually measured frequency characteristic of the coil element is required and thus has been provided from various vendors of coil elements.
In recent years, however, with increasing miniaturization and accuracy of electric circuit products, also with respect to coil elements, there is a growing need for a model thereof further increased in accuracy. For example, Japanese Patent Application Publication No. Hei 11-312187 (the '187 Publication) discloses a highly accurate equivalent circuit capable of successfully exhibiting characteristics of a coil element using a ferrite material. In more detail, as shown in FIG. 12A, there is used a circuit configuration in which a closed circuit composed of a resistance Rm1 and an inductance Lm1 and a closed circuit composed of a resistance Rm2 and an inductance Lm2 are magnetically coupled to an inductance Ls of a LsCpRp parallel circuit at coupling coefficients k1 and k2, respectively. FIG. 12B shows an equivalent circuit expressing a mutual inductance of the circuit configuration shown in FIG. 12A as a coil element. By using such an equivalent circuit, a frequency characteristic of a coil element is reproduced with high accuracy.
However, even the above-mentioned equivalent circuit model according to the background art has presented a problem that a characteristic is not necessarily successfully reproduced in simulation depending on a current or a voltage applied thereto.
FIGS. 13A and 13B each show a comparison between a result of an actual measurement with respect to the equivalent circuit model shown in FIG. 12B performed by using an impedance analyzer and a result of a simulation thereof. FIG. 13A shows a frequency characteristic of an inductance L of a coil element in a case where a minute alternating current of any magnitude is applied thereto. In the figure, a solid line indicates an actual measurement result, and a broken line indicates a simulation result. As shown in this graph, the actual measurement result and the simulation result agree extremely well with each other.
In contrast to this, FIG. 13B shows a result of an actual measurement of the inductance L at a particular frequency in a case where a current is further superposed on the minute current in FIG. 13A and a result of a simulation thereof. As shown in this figure, a simulation value indicated by a broken line does not agree with an actual measurement value indicated by a solid line, and a disparity therebetween increases with increasing amount of the current thus superposed. As described above, accuracy of a simulation decreases with increasing current condition.
FIG. 14B shows a result of a comparison between an actual measurement value and a simulation value of an inductor current IL in a DC-DC converter as shown in FIG. 14A. The DC-DC converter is composed of MOSFETs Qp and Qn, a gate driver GD, a coil element Ld, and a capacitor Cd and uses the above-mentioned equivalent circuit according to the background art as an equivalent circuit for the coil element Ld. In FIG. 14B, a solid line indicates an actual measurement result, and a broken line indicates a simulation result. As is apparent from a comparison between the lines in this graph, an influence of a change in characteristic caused by a current increase or current superposition has not been able to be successfully reproduced on an equivalent circuit model, so that a difference is seen between an actual measurement waveform and a simulation waveform. As described above, according to the background art, sufficient accuracy cannot be obtained also in a transient analysis in which a load current changes on a time axis.